The slotted cylinder antenna, Alford Slot, was first introduced by Andrew
Alford in 1946. Its an omnidirectional horizontally polarized antenna.There
are not many descriptions available, most of them that I have found are
based on the work made by G3JVL in 1978 [Microwave Handbook, M W Dixon
G3PFR].

ALFORD SLOT FOR 1.3 GHz

Due to difficulties to get exactly the right dimensions of materials
in the descriptions, I have chosen to use dimensions easily available in
sweden.
Material, aluminum tube 35 mm outer diam (32 mm inner diam.) can be
bought at P&P (Plat & Profile, Metallvaruhuset, in Gothenburg)
I found it suitable to make it 750 mm long, the slot should be 510 mm long
and 7 mm wide. Turn a top cap and a bottom cap in a lathe and mount a connector
preferably a N-type bulkhead for semi rigid cable UT-141 in the bottom
cap. Make the cable so long that it will reach the middle of the slot and
bend it so it follows the backside of the tube so that it would not disturb
the electrical field in the slot.

I am not using the balun described in the G-land descriptions, I found
that an ordinary lambda/2 balun is easier to make, using UT085 coax. Check
balun performance before mounting it into the Alford. Mount two 100 ohm
SMD resistors (in series) across the 200 ohm side of the balun and check
return loss. I found that the impedance in the feeding point of the
slot is approx. 200 ohms and somewhat inductive. I placed a small capacitor
across the slot to achieve good match, approx. 0,3pF. It is important that
the capacitor is of good quality with low loss e.g. ATC100, ATC180
or something with the similar performance from some other manufacturer
(dielectric labs). RL of the antenna should be in the range 20 -
30 dB.

RADIATION PATTERN

So far I have built and measured three antennas and all of them performs
as expected. I measured the radiation pattern in my garden with some simple
homemade equipment and a PC. I made a comparing measurement against a dipole
to get some approximation of the Alford slot antenna gain.
All measurements done at 1296.8 MHz using a VCO (locked to a 13 MHz
reference), a logarithmic detector from Analog devices AD8313 connected
to an A/D converter. The A/D converter is connected to the PC parallel
port, the antenna rotator is also controlled by the parallel port.